In my pending application, Ser. No. 667,673, filed Mar. 17, 1976, now U.S. Pat. 4,030,895, there is disclosed a reactor for converting a biomass into combustible gases and ash so designed as to effect such conversion without generation of airborne ash and/or the deposit of tar and resins on the grate and in the flues. In the apparatus of said application, the grate is in the form of spaced, parallel bars, the spaces being such that the biomass material to be burned can slide downwardly on the upwardly facing inclined surface without falling through the openings between the grate bars. Primary air is supplied to the downwardly facing side of the grate at a controlled rate so as to pass upwardly through the spaces between the grate bars and the bed of biomass resting thereon to promote a progressive combustion of the mass from the lower end upwardly toward the upper end as biomass is supplied to the upper end to produce CO and ash. The CO is mixed in a mixing chamber at the upper side of the grate with the distillates to provide a combustible fuel and the ash is withdrawn from the lower end of the grate. The objective of this invention is to provide for improved operation of the retort shown in the aforesaid application by maintaining the grate which supports the bed of biomass material at a temperature lower than the fusing temperature of the ash so as to avoid the formation of slag and provide better control of the rate of gas production.